1. Why can the bellows itself "stretch and contract freely"? -Talk through its physical structure
You might wonder, what makes a thin sheet of metal stretch and contract repeatedly like a spring? To put it bluntly, the core is three words:corrugated。 Rolling the flat plate into continuous folds is equivalent to "folding" the deformability of the material into the structure. The peaks and troughs of each ripple are miniature hinges. When the pipe is heated and elongated, these ripples will be slightly flattened; When it cools and shrinks, it springs back.
Bellows do not stretch and contract by the elasticity of the material itself-stainless steel has a high elastic modulus, and what really works isFlexibility due to geometry。 You take a piece of paper and you can't fold it flat, but you can easily compress and stretch it when you fold it into a wavy shape. The same reason. Therefore, the wall thickness of bellows is usually only 0.5~2mm. The thinner it is, the easier it is to deform, but the more it is afraid of corrosion and fatigue.
Our common stainless steel bellows, such as 304 or 316L, will use a multi-layer structure-two, three or even five layers stacked when the wall thickness of a single layer is not enough to bear pressure. There is no welding between layers, and the pressure is transmitted by close fitting, which not only ensures flexibility but also improves the pressure resistance. There is a special formula for calculating the stiffness of this thing, which is related to wave height, wave pitch, wall thickness and number of layers, which will be described in detail later.
2. How are axial, transverse and angular displacements absorbed? Three typical cases are clearly explained
Let's startAxial displacement— — The most common scene: a steam pipe tens of meters long rises from cold to 300℃, with an elongation of hundreds of millimeters. If not compensated, the pipe can bend the bracket or even crack. Axial expansion joints (such as ourUniversal corrugated expansion joint) is specifically for this: the bellows is compressed or stretched along the axis, eating the hot elongation in.
But the pipeline doesn't always go straight. Sometimes the interface between the two devices is misaligned, or the thermal displacement direction changes after the pipeline is turned. At this time, it is necessaryLateral displacementAbsorption. For example: aCompound hinge transverse expansion jointWith two sets of bellows and a hinge structure in the middle, the pipe is allowed to deflect in a direction perpendicular to the axis. In practical cases, the smoke duct of power plant often uses this form to absorb the displacement caused by foundation settlement.
AgainAngular displacement-The thermal expansion of the pipe at the turn will change the elbow angle. At this time, useLarge tie rod expansion jointOrCurved tube pressure balance type expansion joints, which absorb angular displacement in the bending deformation of bellows through tie rods or pressure balance structures. Note that instead of making the bellows twist like a twist, the angular displacement causes it to create an angular change in the bending plane, where one side of the bellows crest is stretched and the other side is compressed.
At this point, you may ask: Can't an expansion energy saver absorb multiple displacements at the same time? Sure, but you need to look at the structural design. Such asStraight pipe pressure balanced expansion jointCan simultaneously absorb both axial and a small amount of lateral displacement at the cost of large size and high cost.
3. Stiffness, pressure and temperature-how do these parameters directly determine the life of the expansion joint?
The lower the stiffness, the stronger the compensation, but the shorter the service life may be。 Why? Because low stiffness means that bellows are softer and more prone to fatigue. When the pressure is high, the corrugated pipe with low stiffness is prone to plane instability-that is, the corrugation can't come back after being flattened, and it is directly scrapped.
ThenpressureWhat about the impact of? We have an empirical data: for every 1MPa increase in working pressure, the fatigue life of bellows decreases by about 30% ~50% (related to the specific waveform). Under high pressure conditions, multi-layer bellows or reinforcing rings must be used, such as oursHigh temperature axial expansion jointIt is often equipped with a built-in reinforcing ring to prevent bellows from bulging.
TemperatureIs a more hidden killer. The creep strength of stainless steel decreases sharply when it is above 400℃. Think about it, when the pipeline runs at 600℃, although the pressure on the bellows has not changed, the material itself has become "soft", and the deformation will continue to accumulate, eventually leading to fatigue cracking of the bellows. At this time, you have to change to a heat-resistant alloy, such as Inconel 625, or useNon-metallic expansion joints (fabric fiber expansion joints)To replace.
An expansion joint with a designed fatigue life of 1000 times may be used for 10 years at normal temperature and low pressure, but if it is placed on a steam pipeline of 400℃ and 1.6MPa, it may leak in two years. Therefore, when selecting the type, don't only look at the displacement, but be sure to report the working condition parameters.
4. Pressure balance type, large tie rod type... What is the difference between the principles of different structures?
Talk firstPressure balanced expansion joint。 The principle is simple: the internal medium pressure will generate a blind plate force on the bellows, and if this force is not offset, it will knock the pipe off. The pressure balanced type allows the axial forces generated by internal pressure to cancel each other by setting another set of bellows (or balancing rings) inside or outside the pipe. OurStraight pipe pressure balanced expansion joint andCurved pipe pressure balanced expansion joints all fall into this category-the difference is that one is used in straight pipe sections and the other is used in bent pipe sections. Note when selecting: The pressure balanced type is usually more than 30% more expensive than the ordinary type, but this money cannot be saved if there are sensitive equipment at both ends of the pipeline (such as steam turbines and pumps).
Look againLarge tie rod expansion joint. Its core is to rely on two thick tie rods to bear the blind plate force generated by internal pressure, and the bellows is only responsible for absorbing displacement. This configuration is particularly suitable for absorbing lateral displacements, such as the flue between the boiler outlet and the dust collector. One end of the pull rod is fixed to one end of the bellows, and the other end slides freely. Adjust the nut position during installation, which can limit the maximum compression amount.
AndExternal pressure single type axial type expansion joint-the bellows is installed in an external sleeve, the medium flows through the inside, but the pressure acts between the inner and outer walls of the bellows? In fact, this structure uses external pressure to stabilize the bellows and prevent internal pressure instability. Suitable for large diameter pipes in vacuum or low positive pressure, such as oursThe vacuum special hose is a similar principle.
As forSleeve-type pipe expansion joints, which rely on sliding seals to compensate for displacement, are completely different from bellows expansion joints. Don't get confused.
5. Can the tie rod be removed during installation? Where does the arrow point? From the principle, you won't be wrong again
When many on-site masters get the expansion joint, their first reaction is to remove the tie rod. Never! The tie rod before installation isTransport support to prevent bellows from deforming during transport. However, after installation in place, the tie rod nut must be loosened (not removed) to allow the bellows to expand and contract freely. If you forget to loosen it, the bellows will be held during thermal expansion, and the tie rod will bend at least, and the bellows will burst at least.
Arrow direction. The expansion joint barrel is usually marked with an arrow marked "flow direction" or "fixed end". Note, however, that this arrow may not coincide with the flow direction of the medium. The arrow points toThe fixed end (dead center) generally points to the side of the pipe fixing bracket. That is, the displacement direction of the expansion joint should be opposite to the arrow. If it is installed backwards, the bellows bears reverse pressure, and the life is directly halved.
One setThe transverse expansion joint of the compound hinge was installed backwards. As a result, the bellows was pressed to the limit during hot operation, and the weld joint was torn, resulting in a loss of more than 100,000. Therefore, be sure to check the position of the fixed bracket and the direction of the arrow on the drawing before installation.
6. Are metal bellows expansion joints the same thing as metal hoses and compensators? Common concept correction
Expansion joints andCompensators are engineered to beSynonyms. The standard term is Expansion Joint, but there are also many in the industry called compensators. In our product list,Non-metallic expansion joints andThe non-metallic compensator is talking about the same thing.
ButMetal hoses andMetal bellows expansion joints are not the same thing. Metal hoses (like oursMetal hoses andPTFE-lined hoses) are essentially flexible pipes that are mainly used to absorb vibration, installation deviations, and compensate for small amounts of displacement. Its bellows are typically longer and softer, but have lower pressure resistance and cannot withstand axial thrust. While metal bellows expansion joints are specially designed to absorb a large amount of thermal displacement, with flanges or takeovers at both ends, higher stiffness, often equipped with attachments such as tie rods and hinges.
Rubber compensators (also called rubber expansion joints) do not fall under the category of metal bellows. It relies on elastic deformation of rubber, and its pressure and temperature resistance are far lower than that of metal expansion joints. It is generally used in low-pressure water systems or fan outlets. Ourrubber compensator andRubber PTFE compensators are of this kind.
Finally,a rotary compensator,Sleeve compensators also compensate for displacement, but the principle is completely different-the rotary compensator relies on hinge rotation, while the sleeve compensator relies on sliding seal. When selecting the model, you must confirm with the customer "What displacement should you compensate for? What is the temperature and pressure?", otherwise it will be easy to be pretentious.